Canada also, at the same time, lifted the remaining restrictions on its airspace to include international, transborder and private flights, but constraints on cargo will remain.

A ban on commercial flights over London remained in effect, although restrictions on light aircraft and helicopter flights was lifted.

TAP-Air Portugal said it will resume its twice-daily flights to the U.S. on Thursday

The decision to close U.S. airspace has resulted in thousands of air passengers being stranded at airports around the world.

But the re-opening of U.S. airports is unlikely to spell the end of the travel chaos facing travellers as airlines face the task of getting aircraft and crews in the correct places before scheduled flights can be fully resumed.

And Canadian Transport Minister David Collenette said: "It is important that Canadian travellers realise that it could take considerable time for flight schedules to get back to normal."

Canada had to take 240 planes and 30,000 passengers diverted from the U.S. at the last minute following the terrorist attacks.

At London's Heathrow Airport, where passengers trying to get to the U.S. and Canada have spent two nights in the terminals, there were no immediate plans for trans-Atlantic flights to depart.

An airport spokesman told CNN: "At this stage we are awaiting further details. We are aware of reports that the U.S. airspace is to re-open but at this stage there are no flights departing here.

"It is fair to say that we expect disruption to continue throughout the day and would advise passengers to contact their airlines before coming to the airport."

As Heathrow reached capacity, with about 2,000 people waiting for flights, a local council gave 140 passengers temporary beds in a hall.

At Gatwick Airport, which on a normal Thursday would have had 33 trans-Atlantic departures and 24 arrivals, a spokesperson said controllers were awaiting confirmation from the U.S. authorities before resuming operations.

Air France said that three flights would leave Paris on Thursday afternoon, two heading for New York and the third to Washington, D.C.

South African Airways (SAA) also said it would resume scheduled flights to the U.S on Thursday.

Passengers at airports around the world have been stranded

"We are happy to announce that the suspension of flights to the U.S. is lifted," Victor Nosi, SAA executive vice-president of marketing and communications, said in a statement to Reuters.

SAA said two flights to New York and one flight to Atlanta were scheduled to leave on Thursday night.

Aaron Munetsi, SAA's regional manager for Nigeria, said Tuesday's postponed flight from New York to Lagos would depart on Thursday afternoon and fly the return leg to New York on Friday amid tightened security.

"We already have adequate security on those flights, but obviously we will adjust our security for the new situation," Munetsi said.

Even when trans-Atlantic flights from the UK resume, airports operator BAA warned of the massive disruption facing passengers.

BAA, which operates a number of UK airports, including Heathrow, Gatwick, Stansted and Glasgow, said that in the immediate future, air travel would be subject to "disruption."

A spokeswoman told the Press Association: "Increases in security, more searches and a stronger police presence at airports -- that is what a passenger can expect."

Europe's transport ministers are due to meet at the weekend to discuss tightening security measures at airports.

The measures are believed to include installing bulletproof glass in cockpit doors, special locking systems to prevent unauthorised access to the cockpit and extra baggage checks, The Associated Press news agency reported.

Passenger flights connecting through London Heathrow, Europe's busiest airport, were already being required to claim their baggage and pass through immigration before rechecking the bags for their connecting flights.